1. Field of the Invention
The present invention generally relates to processes for rebuilding mobile and stationary internal combustion engines. More particularly, the present invention relates to improved apparatus and methods for reconditioning diesel engine cylinder liners.
2. Brief Description of the Background Art
Diesel engines are generally often intended for heavier duty use than, for instance, gasoline engines. Therefore, regarding strength, they are generally overbuilt and moreover, are usually constructed to higher tolerances. This dramatically increases the cost of a diesel engine as opposed to a gasoline engine. Accordingly, it is desirable to enable the owner/operator to recover some of this expense by prolonging the useful life of the engine.
As a result, diesel engines are commonly provided with cylinder liners. The use of cylinder liners can extend engine life by allowing more extensive use of water jackets and coolant passages, thereby providing a cooler running engine. A cooler running engine is further obtained because cylinder liners are generally better heat conductors than the engine block, from which the cylinder walls would otherwise be formed. This is simply because the engine block is formed, for reasons of strength and cost, of cast iron or cast aluminum. In contrast, the cylinder liner need be neither particularly strong nor particularly cheap, and the choice of suitable alloys is therefore not so limited. Accordingly, the liner may be chosen of any appropriate long wearing, heat conductive material. Cylinder liner heat conductivity is still further enabled since the liner may be forged rather than cast, which is not a possibility with the short block.
Cylinder liners are also used in more expensive long-life engines because of their less restrictive maintenance options. In the event that a cylinder wall is badly scored, cracked or chipped, it is often not possible to overbore a block enough to rebuild the engine. This may be because of the lack of availability of the correct size pistons or rings, or simply because of siamesed bore or coolant passage interference. However, it is clear that an otherwise destroyed cylinder liner may simply be replaced.
Additionally, in some engines, it is desirable to coat the cylinder wall with an overlay, or coating in order to reduce friction and further prolong engine life. Even if the cylinder bores of a short block may be overbored, it is often physically difficult to treat or coat those bores afterwards. This is, however, not a problem with removable cylinder liners. Thus, it is seen that not only may the use of cylinder liners per se extend the original life of an engine, but that the liners may themselves be restorable so as to extend the rebuildable life of an engine.
In the past, worn cylinder liners have been restored or reconditioned by simply boring them out as reguired and utilizing oversize piston rings or, if necessary, oversize pistons. As metallurgy became more sophisticated, a two-step boring and plating procedure came into use. The cylinder was first overbored as reguired in order to remove any defects in the cylinder wall. In the event that there were no defects in the cylinder wall, the cylinder was still bored in order to provide sufficient piston clearance for the plating step. After boring, the cylinder wall would be scuffed and the liner immersed in an electrolytic bath in order to apply a chromium overlayer. After this two-step procedure became accepted, certain companies began coating the chrome overlayer with a tetrafluoroethylene (TFE) fluorocarbon polymer (known commercially by the trademark "Teflon") surface. Teflon.RTM. is a product of E. I. du Pont de Nemours & Co.
The Teflon.RTM. provides improved cold weather starting and reduces engine wear, in particular, piston ring wear. Ring wear is reduced both (1) because the Teflon.RTM. acts to seal and fill in the microscopically rough hard chrome surface and (2) because of its native dry lubricity. These factors are especially critical during cold engine start up, when the cylinder walls are provided with no, or at best only minimal pressurized lubrication.
It is apparent that these reconditioning procedures may be repeated as each previously renewed cylinder liner becomes worn yet again. After the cylinder liner is repetitively restored, the aggregate thickness of the individual chrome coatings becomes unwieldy and the cylinder wall surface may start to flake, pit or score, reducing engine life to unacceptable levels. Apparently, this results from the relatively rigid chrome layer being applied on the expansive cylinder material and used in an environment with extreme thermal fluctuations.
In order to reduce the aggregate thickness of the chrome layers, certain companies began to apply a steel base underlay before the chrome surface layer was applied. After the chrome surface was applied on top of the steel base, the Teflon finish was optionally applied. Although this procedure results in a nearly optimal cylinder liner finish, the electrolytic application procedure is itself guite costly and it became clear that a less expensive metal deposition procedure which provides the same results would be desirable.